Accurate fuel gauging in an aircraft plays an important part in the economic operation of the aircraft. If the quantity of fuel on an aircraft can be measured accurately and reliably it enables the minimum amount of fuel to be carried without any risk of danger. In a modern aircraft the weight of fuel can account for half the total weight of the aircraft. By reducing the amount of fuel carried, more passengers or freight can be carried. Alternatively, it can enable the aircraft to have a longer range and reduce the need for refueling stops.
Measurement of the quantity of fuel within an aircraft's fuel-tanks is commonly performed by means of one or more capacitive probes arranged for immersion in fuel in the tank. The capacitance of the probe varies in accordance with the depth of fuel in the tank, thereby enabling an indication of fuel level to be obtained. Changes in permittivity of fuel which would affect the capacitance of the probe can be compensated for by use of a permittivity cell (which may be in the form of a parallel-plate capacitor of open construction) mounted at the bottom of the fuel-tank, so as always to be immersed in any fuel present. An indication of volume may be obtained directly if the tank is of a regular shape, that is, if the volume of fuel present varies in a linear fashion with the depth of fuel. For irregularly shaped tanks the probe may be suitably-shaped such that the surface area of the plates covered by fuel varies in a non-linear fashion with depth but in a manner that is directly related to the volume of fuel present. Alternatively, the output of the probe may be supplied to a computer in which is stored a model of the fuel-tank from which can be obtained an indication of the volume with knowledge of the fuel depth.
In many applications, such as, for example, in aircraft, it is necessary to have an indication of the fuel mass rather than its volume. The mass of fuel can be readily determined by measuring its density with some form of densitometer. Most modern aircraft are equipped with several fuel-tanks and, because of the variations in density between different fuels, such as might be supplied to different tanks during refueling stops at different airports, it is necessary to obtain a measure of the density of each of the fuel mixtures within the different tanks.
Conventional fuel gauging devices typically reside in the aircraft fuel tank as shown in FIG. 1A. More particularly, a conventional aircraft fuel system 10 may include a fuel fill pipe 12 coupled to a fuel tank 14 so as to enable fuel to be delivered to the fuel tank. Mounted to a bottom portion of the fuel tank 14 is a conventional gauging device 16, wherein the gauging device 16 includes a feed pipe 18a for sampling fuel stored in the tank 14 and a discharge pipe 18b for providing the sampled fuel back to the tank. With further reference to FIG. 1B, the conventional fuel gauging device 16 comprises a compensator 20 and a densitometer 22 arranged on a mounting plate 24. The mounting plate 24 is configured for attachment inside a fuel tank 14, such as a lower portion of the fuel tank. The compensator 20 receives a fraction of the fuel delivered to the fuel tank via feed pipe 18a, which is coupled to an input port 20a of the compensator 20. A portion of the fuel delivered to the input port 20a passes through the compensator to output port 20b, which returns the fuel to the fuel tank 14 via discharge pipe 18b. The remaining portion of the fuel is provided to a second output port 20c for delivery to the densitometer 22 as described below. Both the compensator and the densitometer experience relatively low fluid flow rates and are sensitive to the accumulation of gas bubbles that such low flow rates encourage.
The compensator 20 includes a temperature probe 26 for measuring a temperature of the fuel passing through the compensator 20, and a capacitive measurement circuit (not shown) for determining a permittivity of the fuel. The permittivity as determined from the capacitive measurement circuit is corrected based on the measured temperature of the fuel, and a signal corresponding to the measurement is provided at signal terminals 28. Preferably, the compensator is factory-calibrated to be within a known capacitance range.
Moving to the densitometer 22, an input port 22a of the densitometer 22 is coupled to the output port 20c of the compensator 20 via a densitometer feed pipe 30, and an output port (not shown in FIG. 1) of the densitometer 22 returns the fuel to the fuel tank 14. The densitometer 22 includes measurement circuitry (not shown) for measuring the density of the fuel, and a temperature probe 32 for correcting the measured density. A signal corresponding to the fuel density is provided at signal terminals 34. The densitometer also contains a small circuit card containing resistors that represent its calibration data (within a limited tolerance).